A film scanner, as described, for example, in DE 197 31 531 B4, comprises a sensor arrangement for scanning, in a spatially resolving manner, an image zone of the film, such as an arrangement of CCD line sensors, and also driving means which move the film past the sensor arrangement at a controlled speed. During the conversion into a video signal, each image must be converted into a predetermined number of lines of the video signal. Conventionally, the CCD line arrangement is actuated for this purpose using synthetically produced pulses, of which each pulse triggers the scanning of a line, in order thus to scan the specified number of lines within that time period in which an image moves past the sensor arrangement. To this end, the transport speed of the film must be synchronized exactly with the scanning frequency.
In order to produce and maintain such a synchronization, advantage is taken of the fact that the holes of the edge perforation, to be more precise the “Mitchell hole”, represent an exact reference for the position of the images on the film, and a detection apparatus for detecting the perforation holes is used, comprising a light source, a scanning sensor, which is arranged so as to receive light emitted by the light source and transmitted by a perforation zone of the film, and an evaluation circuit for detecting, by means of ah output signal of the scanning sensor, when an edge of a perforation hole passes through between light source and scanning sensor.
This conventional detection apparatus takes advantage of the fact that each time a perforation hole edge passes through the beam path from the light source to the scanning sensor, the light intensity arriving at the scanning sensor experiences a temporary dip. This dip is caused by a change in material occurring in the region of the perforation hole edges during stamping of the perforation holes. In general, such a change negatively affects the mechanical strength of the film material and, with prolonged use, can lead to tearing of the perforation. It is therefore inherently desirable to keep material changes during stamping of the perforation as small as possible. The smaller these material changes are, the weaker, however, is the dip of the light intensity detected by the known detection apparatus when a perforation edge passes through. The weaker the detected dip is, the more difficult it is to distinguish it from fluctuations of the light intensity detected by the scanning sensor which are caused in other ways. Thus the detection of the perforation edges becomes unreliable and imprecise.